B. Podgornik

Tribological properties of plasma nitrided and hard coated AISI 4140 steel

Abstract In the present study, samples made of AISI 4140 steel pre-treated with plasma nitriding and coated with different PVD coatings (TiN, TiAlN and ta-C) were investigated in terms of their microhardness, surface roughness, scratch adhesion and dry sliding wear resistance. Wear tests, in which duplex-treated pins were mated to hardened ball bearing steel discs, were performed with a pin-on-disc machine. To examine the influence of the nitrided zone on the performance of the coating–substrate composite, coatings were deposited on hardened as well as on plasma nitrided samples, prepared under different nitriding conditions. The results of the investigation showed improved mechanical and wear properties of the plasma nitrided hard-coated specimens compared to the uncoated and pre-hardened ones. Furthermore, the compound layer was found to act as an intermediate hard layer leading to superior sliding wear properties of the composite.

Tribological properties of plasma and pulse plasma nitrided AISI 4140 steel

Abstract Plasma nitriding is usually used for ferrous materials to improve their surface properties. Knowledge of the properties of thin surface layers is essential for designing engineering components with optimal wear performance. In our study, we investigated the microstructural, mechanical and tribological properties of plasma- and pulse plasma-nitrided AISI 4140 steel in comparison to hardened steel. The influence of nitriding case depth as well as the presence of a compound layer on its tribological behaviour was also examined. Plasma and pulse plasma nitriding were carried out using commercial nitriding processes. Nitrided samples were fully characterised, using metallographic, SEM microscopic, microhardness and profilometric techniques, before and after wear testing. Wear tests were performed on a pin-on-disc wear testing machine in which nitrided pins were mated to hardened ball bearing steel discs. The wear tests were carried out under dry conditions where hardened samples were used as a reference. The resulting wear loss as well as the coefficient of friction was monitored as a function of load and test time. Several microscopic techniques were used to analyse the worn surfaces and wear debris in order to determine the dominant friction and wear characteristics. Results showed improved tribological properties of AISI 4140 steel after plasma and pulse plasma nitriding compared to hardening. However, the compound layer should be removed from the surface by mechanical means or by decreasing the amount of nitrogen in the nitriding atmosphere, to avoid impairment of the tribological properties by fracture of the hard and brittle compound layer followed by the formation of hard abrasive particles.

Coated machine elements — fiction or reality?

Abstract In the last decade tremendous progress has been made in the field of hard coatings. The majority of research work refers to improvement of mechanical and tribological properties of coatings, deposited on ‘hard’ substrates, i.e. ceramics and tool steels, and used for cutting and forming tools. However, requirements in the case of machine elements are quite different from those for tools. In addition to a hard, wear resistant surface with good frictional characteristics, a tough, fracture resistant core is necessary. In contrast to tool steels, hardened and tempered low-alloy steels have high fracture toughness. On the other hand, high hardness and internal compressive stresses of the case formed by proper thermochemical treatment can lead to increased load-carrying capacity of the steel substrate as well as to improved tribological properties of coated parts. Therefore, the combination of thermochemical treatment and hard-coating, known as a duplex treatment, would allow function sharing between the core material, the hardened case and wear resistant surface, which is of special interest for applications involving machine elements subjected to complex stresses. This paper shows that by proper selection and control of substrate treatment and coating deposition, thin hard coatings could also be successfully used in the case of machine elements subjected to complex stresses.

Influence of substrate treatment on the tribological properties of DLC coatings

Due to the very thin nature of DLC coatings, the substrate must carry the main part of the applied load. If the substrate has insufficient strength to carry the contact load and thus support the coating, plastic deformation will occur, leading to premature failure of the coating. The challenge to improve the properties of hard DLC coatings by thermo-chemical pre-treatment of the substrate has gained much attention in recent years, leading to a new method called duplex treatment. In the present study, a hydrogen-free hard carbon coating deposited on plasma nitrided AISI 4140 steel was investigated with respect to microhardness, residual stress, scratch adhesion and dry sliding wear resistance. The pin-on-disc results showed that nitriding of the substrate improves the wear resistance of the hydrogen-free hard carbon coating as compared to the hardened substrate. The improvement can be related to the increased load carrying capacity of the steel substrate and to improved coating to substrate adhesion.

Wear and friction behaviour of duplex-treated AISI 4140 steel

Abstract In this study samples of AISI 4140 steel were pretreated by plasma nitriding and coated with two different physical vapour deposited coatings (TiN and TiAlN). A hardened AISI 4140 sample and a coated sample were also included in the investigation. To examine the influence of the nitrided zone on the performance of the coating–substrate composite, two different nitriding conditions — a conventional 25% N2 and an N2-poor gas mixture — were used. The specimens were investigated with respect to their microhardness, surface roughness, scratch adhesion and dry sliding wear resistance. Wear tests in which the duplex-treated pins were mated to hardened ball bearing steel discs were performed in a pin-on-disc machine under dry sliding conditions. Metallography, scanning electron microscopy and profilometry were used to analyse the worn surfaces in order to determine the dominant friction and wear characteristics of the samples investigated. The results show improved wear properties of the plasma-nitrided hard-coated specimens compared with uncoated and pre-hardened ones. Although previous investigations showed a negative effect of the compound layer, it was found that a precisely controlled plasma nitriding process can lead to a dense, uniform and highly adherent compound layer with a positive effect on the wear properties of pre-nitrided and hard-coated AISI 4140 steel.

Tribology of thin films and their use in the field of machine elements

Abstract A lot of new technologies and thin films with very good tribological properties, like diamond and diamond-like carbon coatings, were introduced in the last decades. However, use of hard thin films in the field of machine elements is the exception rather than the rule. The main problem lies in the relatively high contact pressure and the very complex loading of the machine components, which demand a hard resistant surface and a tough core. It was found during many numerical and experimental analyses that in the case of hard thin films the plastic deformation of the composite will start in the substrate. Therefore, the ability of the film to sustain the loading depends principally on the load-carrying capacity of the substrate. Although, there is almost an unlimited number of possibilities, we have to be very careful combining different surface treatments and thin-film deposition techniques. It should be pointed out that an improper combination could very easily lead to undesirable deterioration of the properties either of the film or the substrate, instead of their improvement. That is why only a few combinations can be successfully used in practice. One of the most promising ones, already proven in the case of high-speed steel, is the combination of plasma nitriding of the steel substrate followed by the PVD thin-film deposition.

Friction and Lifetime of Laser Surface–Textured and MoS2-Coated Ti6Al4V Under Dry Reciprocating Sliding

The use of lasers for creating defined textured patterns on surfaces has steadily gained attention during the past decade. These textures can contribute to friction reduction by acting as a reservoir for lubricant, hydrodynamic bearing and trap for wear debris. In the present work, titanium alloy surfaces were textured using a nanosecond pulsed laser and subsequently coated with MoS2. The samples were tested under dry reciprocating sliding conditions using two different oscillation amplitudes. During the test, the evolution of the coefficient of friction as a function of the number of cycles was measured until coating failure. The influence of the dimple distance on friction and lifetime was evaluated and verified by post-mortem analyses of the samples using optical, confocal and SEM microscopy as well as EDS and XPS analyses. The results show that under certain conditions, surface texturing can reduce friction, extend the lifetime of the coating and provide a progressive coating degradation until failure.

Influence of Deep-Cryogenic Treatment on Tribological Properties of P/M High-Speed Steel

The aim of the present work was to investigate the influence of deep-cryogenic treatment parameters (treatment time and temperature) on the tribological performance of powder-metallurgy (P/M) high-speed steel. Special emphasis was put on abrasive wear resistance and resistance to galling under dry sliding conditions. Results were evaluated in terms of high-speed steel wear volume, coefficient of friction under reciprocating sliding, friction variation with load, critical load for galling initiation, and stainless steel transfer layer formation. Tribological tests indicate that deep-cryogenic treatment contributes to improved abrasive wear resistance and better galling properties of P/M high-speed steel. However, austenizing temperature is a very important parameter, which defines the level of possible improvement.

Wear properties of induction hardened, conventional plasma nitrided and pulse plasma nitrided AISI 4140 steel in dry sliding conditions

In our study, the friction and the wear behaviour of plasma and pulse plasma nitrided AISI 4140 steel was evaluated under dry sliding conditions, where hardened samples were used as a reference. The nitrided samples were fully characterised before and after the wear testing using metallographic, microhardness and surface examination techniques. After surface characterisation, dry sliding wear tests were performed on a pin-on-disc machine in which hardened ball bearing steel discs were mated to nitrided pins. The influence of sliding speed and contact load on the response of the surface treated pins was determined. The test results indicate, that the wear resistance of AISI 4140 steel can be improved by means of plasma and pulse plasma nitriding. However, compound layer should be removed from the nitrided surface to avoid impairment of the tribological properties by fracture of hard and brittle compound layer followed by the formation of hard abrasive particles.

Residual Stress Field Analysis and Prediction in Nitrided Tool Steel

Residual stresses are present in engineering components as an unintended consequence of manufacturing processes, but they are also deliberately introduced to beneficial effect during surface engineering procedures. Plasma nitriding is a process of particular importance for forming tools and dies, giving significant advantages in wear, and fatigue resistance through the generation of near-surface compressive residual stresses. A precise knowledge of the level and distribution of residual stresses that exist in engineering components is necessary for an accurate design and prediction of a components fatigue resistance. However, measurement of residual stresses is not always possible, which is especially true for forming tools. Therefore, other methods for residual stress evaluation and prediction are required. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase with nitriding time and temperature. On the other hand, experimental data indicates that the residual stress distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution. A minimum in the microhardness depth profile derived over depth corresponds to the location of the compressive residual stress maximum. Furthermore, the residual stress level can be extrapolated by using reference residual stress data and genetic programming. In this way the residual stress level and distribution can be obtained even for components where measurement is not possible.